1. Field of the Invention
The present invention relates to a battery, a method and an apparatus for manufacturing a negative electrode of the battery. More particularly, the present invention relates to stabilization of performance of a non-aqueous electrolyte secondary battery using an active material having a high capacity density, for example, silicon (Si) or a Si compound, for a negative electrode.
2. Background Art
A non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery has received attention as a high capacity power source mainly for portable equipment. Recently, in order to further increase the capacity of this battery, development of electrode materials (use of an active material having a high capacity density and reduction of sub-materials) and improvement (for example, thinning) of mechanical components have been actively carried out.
Especially, silicon (Si), tin (Sn), germanium (Ge) and a compound including such elements, as a negative electrode active material, are a high capacity density material having much higher theoretical capacity than graphite, and the study for using thereof is being carried out. As one example, a non-aqueous electrolyte secondary battery using a thin film of Si, which is formed on a current collector of a copper foil and the like by a sputtering method, as a negative electrode (see, for example, Japanese Patent Application Unexamined Publication No. 2002-83594), and a non-aqueous electrolyte secondary battery using a negative electrode in which an inclined columnar active material including Si is formed on a current collector by a gas phase method (see, for example, Japanese Patent Application Unexamined Publication No. 2005-196970) have been reported.
However, when a negative electrode active material of the compound is formed on a current collector by a gas phase method as in Japanese Patent Application Unexamined Publication No. 2005-196970, the composition of the negative electrode active material varies depending upon its manufacturing conditions. For example, when silicon oxide as a compound of a negative electrode active material is deposited and formed on a current collector by a vacuum vapor deposition method, the composition varies arbitrarily depending upon the amounts of Si and oxygen. When the composition of the negative electrode active material varies in this way, since an amount of lithium ions that can be absorbed by a unit weight of the negative electrode active material is changed, the capacity as a battery becomes unstable. For example, when the composition ratio of Si is reduced, the amount of absorbing lithium per unit weight of the negative electrode active material is reduced, so that the battery capacity is reduced. Furthermore, in this case, the amount of lithium to be absorbed by one atom of Si is relatively increased and lithium that cannot be absorbed at the time of charging may be deposited on a negative electrode as metallic lithium. The deposited metallic lithium is thermally unstable, which may deteriorate the safety. Therefore, it is necessary to prevent the composition of the negative electrode active material from being changed.
However, when a compound of the negative electrode active material is formed on a current collector by a gas phase method, for example, by a vapor deposition method, Si is evaporated from a vapor deposition crucible, the amount of Si as a raw material in the vapor deposition crucible is changed. Accordingly, the vaporization amount of Si is also changed. Thus, it is difficult to keep the manufacturing condition constant in the reactive gas phase method.